For pipe systems in which airborne sound can spread out, as for example in channels of ventilation systems or air conditioning systems, in inlet and outlet pipes of compressors or superchargers, in fresh-gas systems and in exhaust-gas systems of internal combustion engines, silencers are used for prevention of undesirable sound emission to the environment. For example, some silencers operate according to an absorptive design, a reflective design, or combinations thereof. Such designs result in a substantially constant absorbing effect with regards to the absorbed frequencies. However, in the case of internal combustion engines, the spectrum of disturbing sound varies heavily since it is rotation speed-dependent and load-dependent. For example, at dominating engine orders there exist considerably excessive sound pressure levels.
Furthermore, silencers may include a flap with which a gas flow path within the silencer is controllable. There are active systems known in which an external control is required for actuating an actuator driving the respective flap, and passive systems in which the respective flap is actuated by the gas flow. By use of such a flap the absorbing effect and the back pressure behaviour of the silencer can be influenced. However, active systems are comparatively intensive in regard to the manufacturing costs due to the required additional active control components, such as, for example, the control unit, vacuum actuator, vacuum pipe, and control valve. Passive systems are lower priced than active systems, but they can be configured more or less complicated and/or develop only a little acoustic effect and/or generate a comparatively high back pressure and/or have a comparatively high package volume.